1. Field of the Invention
The invention relates to an accelerometer for measuring accelerations along a sensitive axis and, more particularly, to a micromachined vibrating beam accelerometer formed with dual pendulums in one plane which compensates for errors due to cross-axis acceleration without the need for a special mounting structure.
2. Description of the Prior Art
Vibrating beam accelerometers are generally known in the art. An example of such an accelerometer is disclosed in U.S. Pat. No. 5,005,413. Such an accelerometer is formed with a pair of vibrating beam transducers in one plane and configured in a push-pull arrangement such that accelerations along the sensitive axis will cause compression force on one of the vibrating beam transducers and a tension force on the other. The push-pull configuration provides for compensation of various common mode errors, such as vibration rectification errors and certain errors induced by temperature change and drift of the clock frequency. In order to optimize the compensation for the common mode errors, the force transducers must be formed to have nearly identical common mode responses. Various configurations are known for forcing the transducers to have nearly identical common mode responses.
For example, certain accelerometers, such as the accelerometer disclosed in the '413 patent, are formed by micromachining; a technique for fabricating accelerometers from a silicon substrate in a manner similar to the manner in which integrated circuits are fabricated. In order to form the transducers in such an accelerometer in a push-pull relationship, the transducers must be formed either in planes adjacent opposing surfaces of the silicon substrate or in one plane in order to create the push-pull configuration. However, there are various known problems with both alternatives.
In particular, accelerometers formed with vibrating beam transducers in planes adjacent opposing surfaces of the silicon substrate are known to not adequately compensate for common mode errors. The reason for this is that the transducers are formed from different physical layers of the silicon substrate. By forming the vibrating beam transducers in different physical layers of the silicon substrate, the transducers are known to not have well matched common mode responses.
In an attempt to solve this problem, the accelerometer disclosed in the '413 patent is formed with both vibrating beam transducers formed in one plane adjacent one surface of the silicon substrate. By forming both of the vibrating beam transducers in a single plane, the common mode responses of such transducers will be relatively well matched. However, such a configuration creates other problems. For example, such a configuration results in a small angular offset or tilting of the sensitive axis SA (e.g., 6.degree.) which can cause errors in the accelerometer output signal due to cross-axis acceleration. In order to compensate for the tilting of the sensitive axis SA, it is known to mount such accelerometers with a special mounting structure that compensates for the tilting of the sensitive axis.
Although the problem relating to the tilting of the sensitive axis SA can be corrected by utilizing a special mounting structure, there is another problem with such a configuration that is not solved with the use of a special mounting structure. This problem relates to rotation of the sensitive axis SA as a function of the G input. This problem is best understood with reference to FIGS. 1 and 2, which illustrate the accelerometer disclosed in the '413 patent. In particular, the accelerometer 20 is formed with dual vibrating beam transducers 22 and 24 in a single plane adjacent a top surface 26 of the silicon substrate 28. A proof mass 30, formed along the width of the silicon substrate 28, is supported in a plane adjacent a bottom surface 32 of the silicon substrate 28 by a pair of flexures 34 and 36, which define a hinge axis HA. As best shown in FIG. 2, the pendulous axis PA is defined between the center of mass 38 of the proof mass 30 and the hinge axis HA. The center of mass 38 is approximately in the middle of the thickness of the silicon substrate 28 and, since the flexures 34 and 36 are formed along the bottom surface 32, the pendulous axis PA will not be parallel to the plane of the silicon substrate 28. Rather, the pendulous axis PA will be angularly offset or tilted relative to the plane of the silicon substrate 28 by a certain amount as shown, for example, 6.degree.. Since the sensitive axis SA by definition is normal to the pendulous axis PA, the sensitive axis SA will be tilted relative to the plane of the silicon substrate 28 by the same amount.
As the proof mass 30 rotates about the hinge axis HA, the center of mass 38 will likewise rotate. Such rotation of the center of mass 38 will cause rotation of the pendulous axis PA in the plane of the pendulous axis PA and, consequently, will result in rotation of the sensitive axis SA. Such rotation of the sensitive axis SA will be a function of the G input. In some known accelerometers, the rotation of the sensitive axis SA can be on the order of 1 milliradian at maximum G input, resulting in relatively significant errors in the accelerometer output signal.